1. Field of the Invention
This invention relates to the restructuring, by chemical bond rearrangement, of organosiloxane compositions comprising organosiloxane polymers. The object of such restructuring is to change the average molecular weight of the organosiloxane polymers, and, in the most preferred embodiment of the present invention, to provide equilibrated silicone oil compositions.
2. Description of the Prior Art
The desirability of and processes for achieving siloxane bond rearrangement have long been known. The process whereby Si-O linkages of a siloxane or mixture of siloxanes are continuously broken and re-formed until the system reaches an equilibrium condition at the thermodynamically most stable state is generally known as "equilibration". In practice, the term "equilibration" is also sometimes loosely applied to the rearrangement of low molecular weight siloxanes to higher polymers even when the process is not taken all the way to the equilibrium state. While heating alone will bring about this redistribution of siloxanes linkages, equilibrations are usually acid or base catalyzed. Among the catalysts which have been utilized are alkali metal bases such as the lithium, sodium, potassium, rubidium, or cesium oxides, hydroxides, alkoxides, or silanolates, quaternary bases such as the tetraalkylammonium or tetraalkylphosphonium hydroxides or alkoxides, and strong acids such as the Lewis acid HCl complexes, and hydrogen halides, sulfuric acid, boric acid, and trifluoromethylsulfonic acid. A review which gives some indication of the scope and complexity of the status of siloxane bond rearrangement as long ago as 1960 appears on pages 225-264 of the book "Organosilicon Compounds" by E. Eaborn, Academic Press, Inc., New York.
The utilization in siloxane equilibrations of catalysts such as those mentioned above suffers from a serious disadvantage. In order to obtain neutral high quality fluids, such catalysts require either extensive neutralizations and filtrations or thermal deactivation or destruction of the catalyst. It has previously been found that the use of supported catalysts avoids some of those catalyst post-treatment problems. Britton et al. U.S. Pat. No. 2,460,805 (February 8, 1949) was the first U.S. Patent to issue claiming the use of acid clays as catalysts for siloxane equilibrations. Knopf et al. U.S. Pat. No. 2,831,008 described siloxane equilibrations catalyzed by the use of fine-grained dilute acid-treated cation exchange media, such as carbon kaolin, montmorillonite/quartz, charcoal, bleaching earth, and gel-type synthetic resins. Eynon U.S. Pat. No. 3,322,722 taught that siloxane-alkoxy/acyloxy interchange in batch reactions was catalyzed by strong acids such as sulfuric acid and sulfonated macroreticular cation exchange resins. Merrill U.S. Pat. No. 3,375,223 showed that silanol condensation was catalyzed by a hydrogen chloride activated particulated siliceous material such as diatomaceous earth and fumed silica. Steward U.S. Pat. No. 3,398,177 showed redistribution of Si-H bonds catalyzed by acid clays. Litteral U.S. Pat. No. 3,694,405 described a continuous siloxane equilibration catalyzed by a macroreticular sulfuric acid cation exchange resin. Miller et al. U.S. Pat. No. 3,714,213 described how cyclopolysiloxanes containing methyl and silanic hydrogen substituents were prepared by catalytically cracking and cyclizing polysiloxanes containing Si-CH.sub.3 and Si-H groups and high molecular weight chain terminal groups. The catalysts employed were acid-treated clays or synthetic alumina silicates. Nitzsche et al. U.S. Pat. No. 3,816,493 described the continuous preparation of organopolysiloxanes in an upflow design reactor using acid clays and sulfonated styrene-divinylbenzene copolymer. Sicilliano U.S. Pat. No. 3,853,933 taught that polysiloxane oils could be prepared continuously by siloxane-equilibration utilizing an acid-activated carbon black catalyst bed. Finally, Sicilliano et al. U.S. Pat. No. 3,853,934 showed that polysiloxane oils could be prepared continuously by siloxane equilibration utilizing an acid-activated clay column.